78 



MINERALOGY 



CLASS, HEMIHEDRAL HEMIMORPHIC 

 TYPE 21, TETRAGONAL POLAR 



Symmetry. There is one axis of tetragonal symmetry ; all the 

 symmetry of the equatorial plane is lost. This type is related to 



the tetragonal equatorial in the same 

 way as the ditetragonal polar is to the 

 ditetragonal equatorial, Fig. 143. 



Forms 

 -: i Tetragonal hemipyramid of the third 



order, u/1 



na : a : me 



ir(hkl). 



FIG. 143. Type 21: Tetragonal 

 Polar. 



The tetragonal pyramid of the third 

 order yields the only new forms of the 

 type, the positive, Fig. 144, and nega- 

 tive upper and lower hemipyramids of 

 the third order. 



Forms possible to combine in the type are : 

 upper and lower hemipyramids, of the third order, IT (hkl) 

 ir(khl), ir (hkl), ir(khl). 



Upper and lower hemipyramids of 

 the second order, (ohl), (ohl). 



Upper and lower hemipyramids of 

 the first order, 

 (hhl), (hhl). 



prisms of 

 the third order, 

 IT (hko), IT (kho). 



Prism of the second order, (oho). 

 Prism of the first order, (hho). 

 Basal pinacoid, (001), (001). 

 Examples. Wulfenite, PbMo0 4 , (111) (111) 

 (001) (430), Fig. 145. 



Etch figures. In the seven types of the 

 (ill) (ill) (ooi) (430). tetragonal S y S tem it will be noted that two 



forms, the prisms of the first and second orders, are common to all. 

 They are of the same shape and possess the same number of faces 

 in each type ; and in external appearance the prism of one type is 

 not to be distinguished from that of another. The same condition 

 exists in the case of the cube and rhombic dodecahedron in the five 



FIG. 144. The Upper Hemipyra- 

 mid of the Third Order, n-(hkl). 



FIG. 145. Wulfenite, 



